title: Maintaining tumor targeting accuracy in real-time motion compensation systems for respiration-induced tumor motion.
abstract: OBJECTIVE
  To determine how best to time respiratory surrogate-based tumor motion model updates by comparing a novel technique based on external measurements alone to three direct measurement methods.
  
  
  METHODS
  Concurrently measured tumor and respiratory surrogate positions from 166 treatment fractions for lung or pancreas lesions were analyzed. Partial-least-squares regression models of tumor position from marker motion were created from the first six measurements in each dataset. Successive tumor localizations were obtained at a rate of once per minute on average. Model updates were timed according to four methods: never, respiratory surrogate-based (when metrics based on respiratory surrogate measurements exceeded confidence limits), error-based (when localization error ≥ 3 mm), and always (approximately once per minute).
  
  
  RESULTS
  Radial tumor displacement prediction errors (mean ± standard deviation) for the four schema described above were 2.4 ± 1.2, 1.9 ± 0.9, 1.9 ± 0.8, and 1.7 ± 0.8 mm, respectively. The never-update error was significantly larger than errors of the other methods. Mean update counts over 20 min were 0, 4, 9, and 24, respectively.
  
  
  CONCLUSIONS
  The same improvement in tumor localization accuracy could be achieved through any of the three update methods, but significantly fewer updates were required when the respiratory surrogate method was utilized. This study establishes the feasibility of timing image acquisitions for updating respiratory surrogate models without direct tumor localization.
authors.affiliation: Fischell Department of Bioengineering, A. James Clark School of Engineering, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Kathleen
authors.initials: K
authors.lastname: Malinowski
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: McAvoy
authors.forename: Rohini
authors.initials: R
authors.lastname: George
authors.forename: Sonja
authors.initials: S
authors.lastname: Dieterich
authors.forename: Warren D
authors.initials: WD
authors.lastname: D'Souza
mesh_terms: D057918:Fiducial Markers; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000503:physiopathology* / Q000532:radiotherapy*; D008954:Models, Biological; D009068:Movement; D010190:Pancreatic Neoplasms / Q000000981:diagnostic imaging / Q000503:physiopathology* / Q000532:radiotherapy*; D011859:Radiography; D012119:Respiration; D012189:Retrospective Studies; D013997:Time Factors